Combustion

Combustion is the reaction of substances with oxygen, with the evolution of heat and light.

The combustion of organic materials are often free radical chain reactions, which can usually be summarised as the oxidation of carbon content of the material to form its oxides and the oxidation of hydrogen to form water.

The corrosion of metals is the result of the slow combustion of the metals with the oxygen in air.

Combustion of Metals


Combustion of Carbon

A piece of wood charcoal, which is an impure form of carbon, when strongly heated in a deflagration spoon burns brightly, and then placed in a gas jar containing oxygen, burns even more vigorously throwing off bright sparks.

The chief product of the reaction is carbon dioxide, although a little carbon monoxide is also formed :


                    	C   +   O2   ==>   CO2  	
			           Carbon Dioxide	

	               2C   +   O2  ==>	CO  	
			       Carbon Monoxide	
When shaken with water, the Carbon Dioxide, CO2, dissolves forming a solution of the very weak and unstable solution of Carbonic Acid, H2CO3, which changes the blue colour of litmus to a port-wine red colour :

		CO2   +   H2O   ==> H2CO3  	
		Carbon		  Carbonic	
		Dioxide		     Acid	
If another jar of containing Carbon Dioxide, CO2, from the combustion of carbon is shaken with lime water, this becomes milky from the formation of a precipitate of Calcium Carbonate, CaCO3.

		Ca(OH)2   +   CO2   ==>	CaCO3   +   H2O	
		Calcium			Calcium	
		Hydroxide		Carbonate	


Combustion of Hydrogen

When a jet of hydrogen gas that is prepared by the action of dilute sulphuric acid on zinc burns in a dry oxygen water is produced and it condenses as a dew on the inside walls of the jar.

		2 H2   +   O2   ==>   2 H2O  


Combustion of Metals

Some metals, when heated strongly in oxygen, burn to form metal oxides. These oxides are the Basic Oxides.

Other metals do not burn readily in oxygen, but react with it on heating to form Oxides.


Combustion of Sodium

When sodium is heated in an iron deflagrating spoon until it ignites and then lowered in dry jars of oxygen, it burns with bright yellow flame forming orange-yellow solid higher oxides (e.g. sodium peroxide, Na2O2).

		2 Na   +   O2   ==>             Na2O2       
                                                                              Sodium  
                                                                              Peroxide        
When this substance is dissolved in water, oxygen is evolved and an alkaline solutions, containing sodium hydroxide, which turn red litmus blue is formed.

            2 Na2O2  +  2 H2O ==>         4 NaOH   +   O2 
            Sodium                                       Sodium  
            Peroxide                                     Hydroxide       

Combustion of Potassium

When potassium is heated in an iron deflagrating spoon until it ignites and then lowered in dry jars of oxygen, it burns with bright lilac flame forming orange-yellow solid higher oxides (e.g. potassium peroxide, K2O2, and potassium dioxide, KO2).

                                2 K   +   O2  ==>      K2O2       
                                            Potassium       
                                              Peroxide        

                                  K   +   O2       ==>    KO2     
                                           Potassium       
                                              Dioxide 
When these substances are dissolved in water, oxygen is evolved and an alkaline solutions, containing potassium hydroxide, which turn red litmus blue is formed.
>
            2 K2O2  +  4 H2O ==>   4 KOH   +   O2  
            Potassium                       Potassium       
            Peroxide                           Hydroxide       

            4 KO2   +   2H2O ==>   4 KOH   +   3 O2        
            Potassium                       Potassium       
            Dioxide                         Hydroxide       

Combustion of Magnesium

When magnesium ribbon held in a crucible tongs is ignited in air and inserted into a jar of oxygen, it burns with a blinding white light, forming white solid magnesium oxide, which is a sparingly soluble basic oxide and turns moist red litmus paper blue.


Combustion of Calcium

When heated in an iron deflagrating spoon, calcium burns brightly in oxygen forming calcium oxide, which turns moist red litmus blue.


            2 Ca   +   O2   ==>   2 CaO     
                                  Calcium Oxide  

            CaO   +   H2O  ==>   Ca(OH)2  


Combustion of Iron

When a spiral of iron wire tipped with a piece of burning wood, is lowered into a gas jar containing oxygen gas, the iron burns brilliantly, giving off bright sparks of the burning metal. Ferrosoferric oxide, Fe3O4, is formed in fused globules, which fall to the bottom of the gas jar, on a layer of sand. It has no action on litmus.


                             3 Fe   +   2 O2  ==>   Fe3O4   
                                              Ferrosoferric        
                                                    Oxide        

Ferrosoferric oxide may be regarded as a compound of ferrous oxide and ferric oxide and it is sometimes called a mixed oxide.


            FeO   +   Fe2O3   ==> Fe3O4     

Combustion of Non-Metals in Oxygen

The chemical characteristic of the non-metals is that their oxides on reaction with water give rise to acids. The acidic properties of the hydrolysis of these oxides vary considerable, so that the oxides of sulphur and phosphorus give rise to strong acids, while the oxides of boron give rise to weak acids.

Some non-metallic elements, such as phosphorus, sulphur, and carbon, burn in oxygen to form Acidic Oxides


Combustion of Phosphorus

When a piece of phosphorus on an iron deflagrating spoon is kindled by touching it with a hot wire, and it is put into a gas jar containing oxygen, it burns in the oxygen with an exceedingly brilliant white flame, producing a white cloud of phosphorus pentoxide, P2O5, which settles in flocks on the inside of the dry jar.

            P4   +   5 O2  ==>   2 P2O5    
When water is poured into the jar the oxide dissolves and phosphoric acid is formed, which changes the colour of blue litmus solution to red :

            P2O5  +  3 H2O ==>  2 H3PO4      

Combustion of Sulphur

When a small amount of sulphur is kindled on a deflagrating spoon, it burns with a bright blue flame when introduced into a gas jar containing oxygen. A gas, sulphur dioxide is the main product of the combustion. However, a little sulphur trioxide is also formed, which makes the gas slightly cloudy.


            S   +   O2  ==>        SO2     
                                    Sulphur Dioxide 

            2 S   +   3 O2  ==>   2 SO3    
                                    Sulphur Trioxide        

When shaken with water, the products of combustion dissolve, forming an acidic solution which turns litmus red.


            SO2   +   H2O   ==>     H2SO3   
            Sulphur                       Sulphurous      
            Dioxide                             Acid    

            SO2   +   H2O  ==>   H2SO4     
            Sulphur                       Sulphuric       
            Trioxide                        Acid    

Start of Hypertext .... Elements .... Compounds .... Index
Hypertext Copyright (c) 2000 Donal O'Leary. All Rights Reserved.